For the purposes of setting the desired operating voltage, the necessary number of fuel cell units are arranged one upon the other in order to obtain a fuel cell pile (fuel cell stack). In order to prevent an electrical short-circuit, the housings of the successive fuel cell units in the fuel cell stack must be electrically insulated from one another. Moreover, it is necessary to separate the fuel gas channels of the fuel cell stack from the oxidizing agent chambers of the fuel cell units in gas-tight manner and to separate the oxidizing agent channels of the fuel cell stack from the fuel gas chambers of the fuel cell units in gas-tight manner.
In the case of known fuel cell stacks, sealing and insulating elements consisting of a glass solder or of ceramic sealing materials are used in order to produce the requisite electrically insulating effect and the requisite sealing effect.
In the case of most of the usually used sealing materials, the electrical resistance at the operating temperature of a high temperature fuel cell unit (in the range of approximately 750° C. to approximately 850° C.) is no longer high enough for achieving a satisfactory insulating effect. Furthermore, some of the usually used sealing materials only exhibit a low level of chemical stability and mechanical rigidity for the changes in temperature (between the operating and quiescent phases) that frequently occur in a high temperature fuel cell unit.
The sealing function and the electrically insulating function of the sealing arrangement can be separated from one another. Thus, the electrical insulation can be effected by a ceramic coating which is connected to an adjacent component of the fuel cell stack by means of a metallic solder in the course of a soldering process. Hereby, due to the gas-tight soldering process, the respective processes of sealing the fuel gas channels and the oxidizing agent channels of the fuel cell stack and the mechanical fixing of the fuel cell units to one another is effected at the same time.
The insulating ceramic coating can, however, contain pores and/or fissures and/or capillaries along the grain boundaries, especially if the ceramic coating is applied by a process of thermally spraying it onto a metal part that is to be insulated. Depending upon the capillary activity of the solder used, the solder can penetrate into the pores, fissures of capillaries present in the ceramic coating and cause a short-circuit.